Machine to machine (M2M) networks and devices are increasingly growing in number and are projected to outnumber existing cellular networks. However, existing personal area networks (PAN) standards such as IEEE 802.15.4 and Zigbee are not equipped to handle such a large amount of traffic, particularly whilst increasing communications range and reducing power. These networks are expected to cover a wide variety of applications ranging from smart energy meters, temperature/traffic monitoring, body area networks and industrial automation to name but a few examples.
One important consideration in machine to machine communications is the battery life of sensor nodes or indeed other resource limited nodes in the network. It is desirable that battery lifetimes of sensor nodes for machine to machine communications should be of the order of years as opposed to days, as seen in cellular communications, and less than 1 to 2 months in current personal area networks and machine to machine standards.
In view of this, sensor nodes are required to have a low duty cycle for these various operating modes and remain in sleep mode for most of the time. In sleep mode, most of the transceiver components of a sensor node, such as the digital transmission blocks, power amplifiers, receiver chain and micro-processor are turned off. In order to transfer information to the network, sensor nodes occasionally transition from sleep mode to active transmit and receive modes and communicate data to and from the network.
One technique for controlling the duty cycle of a sensor node is to synchronize sensor nodes to periodically wake up from a sleep mode to an active mode. In active mode, the sensor node scans the physical network and enables information transfer to and from the network. This technique is disadvantageous as the sensor node will periodically wake up from sleep mode to active mode, regardless of whether it has any information to transmit/receive, and it may involve a sensor transitioning to an active state to serve no function.
Another technique for controlling the duty cycle of a sensor node is to control the sensor node so that it remains in an idle state and then transition to an active mode in which the sensor scans the physical network and enables information transfer to and from the network. The idle state is a state in which the transmit/receive functions are turned off and are not being used but their controller is powered such that it can be readily used. Therefore, in idle mode, the sensor node consumes less power than in a transmit/receive modes, but it consumes more power than when in sleep mode.
A further technique for controlling the duty cycle of a sensor node is to periodically wake up a sensor node. Depending on events from the sensor or co-ordinator, transmission with the network can be controlled. This is disadvantageous as it requires the sensor nodes to wake up regardless of whether there is anything to sense, thereby potentially wasting power.
In a still further technique, neighbouring sensor nodes can co-ordinate their duty cycles.
All of the above techniques require sensing and acknowledgement of the wireless channel to achieve communication, which in turn consumes a significant amount of energy. Therefore, sensor nodes using the above techniques will be limited to battery lifetimes of the order of 1 to 2 months. In particular, in medium to large networks, the sensor nodes may expend significantly more energy in sensing and sleep cycle management when compared to communication updates with the network.
It is therefore desirable to provide mechanisms to optimise the sleep duty cycle of sensor nodes.